EP4176095A1

EP4176095A1 - Process for the simultaneous treatment of residues of the non-ferrous metallurgical industry to produce pigments based on iron oxides and other valued products, in accordance with circular economy strategies

Info

Publication number: EP4176095A1
Application number: EP21749719.7A
Authority: EP
Inventors: Aldo Imerito; Mario Ragona; Massimo Pisu; Pierandrea AZARA
Original assignee: Ecotec Gestione Impianti SRL
Current assignee: Ecotec Gestione Impianti SRL
Priority date: 2020-07-03
Filing date: 2021-07-05
Publication date: 2023-05-10
Also published as: WO2022003747A1; IT202000016192A1; WO2022003747A4

Abstract

Described is a process for the simultaneous treatment of residues of the non-ferrous metallurgical industry, in particular Waelz slag and residues deriving from thermal fuming processes of jarosite, goethite or KSS slag, comprising the following steps: - possible initial pre-treatment (A) of the material to be treated, to obtain homogeneous particle size characteristics; - thermal pre-treatment (B) by the addition of ammonium sulphate (or nitric acid and/or sulphuric acid mixed with ammonium sulphate or with another reagent capable of promoting a reaction with the Waelz slag which makes it possible to obtain firstly the formation of the reaction intermediate ammonium hydrogen sulphate, (NH4)HSC>4 and, subsequently and in sequence, the formation of soluble iron and manganese sulphates and the development of ammonia) and gradual increase in temperature from ambient temperature to a final temperature in the range 200 °C to 600 °C, in a neutral/oxidising environment, to obtain the conversion of the iron, and manganese if present in the material, into compounds such as iron sulphates, and, possibly, manganese sulphates, soluble in water, with the formation of ammonia; and - hydrometallurgical treatments (C), according to the following sub- steps: - addition of water and mixing, - correction of acidity up to a pH of 2 to 8 in an oxidising environment, to obtain the precipitation of the iron in the form of mixed oxyhydroxides. The invention also relates to the use of the precipitate thus obtained, after separating from the solution and purification, for the production of pigments based on iron oxide.

Description

Process for the simultaneous treatment of residues of the non-ferrous metallurgical industry to produce pigments based on iron oxides and other valued products, in accordance with circular economy strategies The invention relates to the technical field of the treatment of solid residues generated by the non-ferrous metallurgical industry, with the aim of producing valued products and secondary raw materials from them and drastically reducing or eliminating the need to dispose of these residues in landfills, in accordance with circular economy strategies. In particular, the invention relates to a simultaneous treatment of various types of residues generated by the zinc and lead production industry.

The current management of the residues generated by the zinc and lead production industries is environmentally, economically and socially highly problematic and requires action in the short and medium term. Some of these residues still contain, to a greater or lesser extent, potentially valuable elements but in a chemical form unsuitable for extraction by commonly used industrial processes (jarosite, goethite, KSS slag, Waelz slag, residues from thermal fuming of jarosite, goethite or KSS slag). In particular, Waelz slag and residues from thermal fuming processes of jarosite, goethite or KSS slag, as well as traces of minority elements in combined form, mainly have a significant content of oxide iron in a low oxidation state, but uniformly distributed within a calcium-aluminium-silicate matrix. Moreover, since Waelz slag and residues from thermal fuming processes of jarosite, goethite or KSS slag leave the plant in molten form at a temperature of about 1000°C and are suddenly quenched by wet granulation, the matrix has a strong amorphous component.

This chemical form makes the iron oxides inseparable from the matrix by mechanical means and magnetic separations; on the other hand, precisely because of these characteristics, Waelz slag and the residues deriving from thermal fuming processes of jarosite, goethite or KSS slag are chemically inert and therefore easily managed as waste. However, if one considers, for example, that an average-sized Waelz plant produces about 150,000 t/year of slag, this leads to a significant annual consumption of landfill volumes and a loss of the potential value of the iron oxides present.

This conduct leads to a loss of valuable materials and irrational use of available landfill volumes, which are becoming increasingly scarce. In addition, in view of the environmental policies linked to disincentives for the granting of new landfill space, there is a risk that, in the near future, industries will reach saturation point in this disposal route, resulting in the need for closure and the loss of thousands of direct and indirect jobs.

In recent years, the zinc and lead production sector has also become aware of the need for a shift from the Linear Economy to the Circular Economy. For this reason, new processes are being studied to reduce the amount of waste produced at source and, where this is not possible, to identify techniques and treatments capable of minimising the amount of waste to be sent for disposal, recovering and reintroducing into the production cycles as many of the components as possible that can be used. To achieve this, new processes are being developed specifically for waste currently destined for landfill.

On potentially "rich" residues, such as jarosite, goethite and KSS slag, various possibilities of separating compounds and reintroducing them into production cycles have been identified; for Waelz slag and, sometimes, for residues deriving from the thermal processes of "fuming" jarosite, goethite or KSS slag, the main possibilities of reuse known in the literature concern their use as aggregates for civil works or recovery of the iron by means of thermal reduction. With regard to the first type of use, when Waelz slag was used in the past as such for road sub-bases, in contact with the weather conditions it gave rise to contamination due to the slow but constant release of metals.

With regard to the use in cementitious matrices, or in the production of ceramic materials, this possibility is currently only theoretically feasible, as the relative studies are still in progress and real applications are still rare.

The separation of iron in the form of cast iron by high-temperature carboreduction is known and possible by various processes. However, as there is often enough copper present along with the iron to contaminate the cast iron produced, making it unmarketable, this type of recovery is not universally applicable, but only when copper is absent or present in a negligible concentration.

Given the intrinsic value of recoverable iron and the cost of processes to selectively extract the copper and iron, this option, although technically feasible, is not always economically viable.

Therefore, in this specific sector, there is a need to fill the gaps due to the current state of the technologies applicable to the treatment of Waelz slag and residues deriving from thermal processes of "fuming" of jarosite, goethite or KSS slag for the production of new secondary raw materials, in view of the Circular Economy.

This is the background to the solution according to the invention, which aims to provide an articulated process based on thermal and hydrometallurgical processes.

In fact, the process according to the invention offers numerous advantages, which will become clear below.

The process according to the present invention is directed to the simultaneous treatment of various types of residues generated by the zinc and lead production industry.

A general description is given below of the process according to the invention, describing, by way of a non-limiting example, the application to the treatment of Waelz slag. However, the process according to the invention may be applied to other types of starting materials, such as, for example, residues deriving from thermal fuming processes of jarosite, goethite or KSS slag having an content of iron oxide distributed in a comparable manner, within a calcium aluminium silicate matrix sufficient to make the process technologically and economically sustainable. Figure 1 and Figure 2 attached schematically illustrate, by way of non-limiting example, a block diagram of the process according to the invention, applied to the treatment of Waelz slag.

The proposed process uses thermal and hydrometallurgical treatments and is conceptually divided into three steps:

- mechanical pre-treatment of the material;

- thermal pre-treatment of the material; and

- hydrometallurgical treatments.

The process according to the invention, which will be described in detail below by way of a non-limiting example with particular reference to preferred embodiments, is capable of selectively extracting the iron present in the material, in order to obtain a ferrous material that can be converted into high value-added products. In particular, according to the invention, it is preferably proposed to convert the iron material obtained into oxide-based pigments, hereinafter identified by the acronym IOP (Iron Oxide Pigments), whose commercial value makes the cost of the treatment process itself self- sustainable. Consider, for example, that one tonne of Waelz slag can yield approximately 0.25 tonnes of IOP and that the minimum price for this product is approximately EUR 1 ,400.00 per tonne of IOP. The valorisation of this product, for an annual production of about 130,000 t/year of Waelz slag, is potentially estimated at 45 million euros.

In addition, the process according to the invention makes it possible to extract the manganese present in the residue by means of extraction and purification processes of known type, such as, by way of example, solubilisation in bases and subsequent electrochemical deposition, obtaining manganese oxide for use in the reference market.

The proposed process also allows the selective recovery of the silver present in the residue through known extraction and purification processes such as, for example, extraction with saline solutions and precipitation with reagents, obtaining silver sulphide to be sent to the reference market or to further refining processes.

The proposed process enables the regeneration and recovery of material flows within the process itself. This feature allows the reactive materials to be recirculated within the process, with the minimal reintegration required, while simultaneously reducing overall energy consumption and environmental impact, further increasing the self sustainability of the process according to the criteria of the Circular Economy.

At the end of the separation steps, the proposed process also generates a chemically inert material, based on calcium/aluminium silicates, suitable for use as a secondary raw material in civil engineering and construction works. In the less favourable case that the market is unable to fully absorb this material, the remainder complies with the requirements for disposal in landfills, without further treatment.

The proposed process, as a whole, is therefore able to produce, from the waste originally destined for landfill, compounds with high added value destined for the reference markets (IOP, Iron Oxide Pigments), manganese, silver, aggregates for use in civil engineering and construction works), thus reducing the potential waste to be disposed of. In a less favourable scenario, where reuse of the inert fraction is not possible for market reasons, it can be easily disposed of; even in this unfavourable case, the reduction of waste to be disposed of, compared to the situation with the currently known processes, would be at least 50%.

The proposed process, which will be described in more detail below (the capital letters in brackets refer to the process steps shown in Figures 1 and 2), employs thermal and hydrometallurgical treatments and is conceptually divided into three steps:

- initial pre-treatment (A);

- thermal pre-treatment of the material (B); and

- hydrometallurgical treatments (C).

In particular, the process according to the invention can be applied to cooled and heap-bound Waelz slag, to newly produced Waelz slag taken directly at the plant mouth or to mixtures of the two materials.

The initial pre-treatment (A) according to the proposed process involves either a grinding of the heap-bound Waelz slag (which generally has a particle size optimised for heap-bound slag that is too large for treatment according to the invention) or a cooling of the newly produced Waelz slag taken directly at the plant mouth. If a mixture of the two materials is used, they can be mixed before the specific pre-treatment or after the pre treatment of the individual materials. The thermal pre-treatment (B) according to the process of the invention is able to obtain a matrix predominantly based on highly water- soluble saline compounds; this transformation serves to give the Waelz slag thus treated specific characteristics to favour subsequent leaching reactions, making the selective recovery of chemicals of interest more efficient.

In the process according to the invention, the thermal pre-treatment (B) is carried out by adding ammonium sulphate (or nitric acid and/or sulphuric acid in a mixture with ammonium sulphate or another reagent) capable of promoting a reaction with the Waelz slag which allows firstly the formation of the reaction intermediate ammonium acid sulphate, (NH4)HSC>4 and, subsequently and in sequence, the formation of soluble iron and manganese sulphates and the development of ammonia. According to a preferred embodiment, in the mixture obtained using ammonium sulphate, the ratio of this reagent to Waelz slag is between 10% w/w and 50% w/w. If nitric acid and/or sulphuric acid are used in a mixture with ammonium sulphate, the resulting reagent mixture will be mixed with the Waelz slag at a ratio of between 10% w/w and 50% w/w. For simplicity of description, the process according to the invention will be described below using ammonium sulphate as a reagent, it being understood that a mixture consisting of nitric acid and/or sulphuric acid in a mixture with ammonium sulphate may be used in the process, without this leading to changes in the steps of the process.

The Waelz slag mixed with ammonium sulphate (or with the mixture consisting of nitric acid and/or sulphuric acid mixed with ammonium sulphate) is heated to a temperature of between 200 °C and 600 °C, using any apparatus or set of apparatuses capable of ensuring neutral/oxidising reaction conditions and working temperatures of between 200 °C and 600 °C. By way of non-limiting example, thermal pre-treatment (B) may be carried out by means of electric furnaces, direct fired and indirect fired rotary kilns, diathermic oil or molten salt fluid exchange furnaces, microwave ovens or combinations thereof.

In the process according to the invention, during the thermal pre- treatment (B), the Waelz slag combined with ammonium sulphate is subjected to a thermal treatment in a reactor, in a neutral/oxidant environment, at temperatures increasing from ambient temperature up to a final temperature of between 200 °C and 600 °C. The temperature ramp is not significant for the purposes of the process and depends solely on the industrial machinery with which this step is carried out and the hourly capacity of the machinery. Indicatively, however, it is possible to say that, from the point of view of completion of the reaction, the process in which the material has to be subjected to temperatures of 200 °C to 600 °C preferably lasts about 1 hour, overall).

The thermal pre-treatment described above enables the conversion of the iron and manganese present in the Waelz slag into compounds such as iron sulphates and manganese sulphates that are water-soluble and therefore more leachable in the subsequent extraction and purification steps. In the temperature range of 200 °C to 600 °C, the ammonium sulphate reagent used (or the mixture of nitric acid and/or sulphuric acid mixed with ammonium sulphate) decomposes to form ammonium acid sulphate and ammonia. The ammonium acid sulphate reacts with the iron and manganese oxides to form the respective sulphates and water. At the temperature of between 200 °C and 600 °C, the ammonia and water are removed from the reactor on the off-gas line and subsequently condensed, generating ammonia in concentrated solution. The ammonia thus obtained is then stored and used as a reagent and as an acidity corrector in the subsequent steps of the process. At the end of the thermal pre-treatment (B) of the Waelz slag with ammonium sulphate, a flow of recoverable ammonia and a solid consisting mainly of iron and manganese sulphates is obtained which will undergo the hydrometallurgical treatments (C).

In the process according to the invention, for the initiation of the hydrometallurgical treatment step (C), the solid resulting from the previous heat treatment step (B) of the Waelz slag, also referred to as roasting, is then directed to the extraction processes. In particular, the material is placed in contact with water and agitated, with the aim of promoting the passage of iron and manganese sulphates into solution.

The mixing of the pre-treated Waelz slag with water can be carried out industrially with any machine or equipment of a type known to the sector technology. The solid washed with water is then separated from the liquid and conveyed to a subsequent process step dedicated to the recovery of silver (G), which will be described below. The liquid, on the other hand, selectively enriched with iron and manganese, continues the hydrometallurgical treatment. In particular, under the reaction conditions according to the invention, the washing liquid of thermally pre-treated Waelz slag is subjected to an acidity change of between pH 2 and 8 in an oxidising environment (D). According to a preferred version of the process according to the invention, the pH change is achieved by the addition of ammonia from the previous thermal pre-treatment step. According to a preferred embodiment of the invention, the oxidising environment may be obtained by insufflation of oxygen, air, oxygen-enriched air. The change in pH and the oxidising environment cause precipitation of the iron initially dissolved in the liquid in the form of mixed oxyhydroxides. According to a preferred version of the invention, the precipitation of iron can take place at a temperature of between 10°C and 150°C, with a partial oxygen pressure of between 0 bar and 10 bar and a contact time of between 1h and 4h. The precipitate thus obtained is separated from the solution and sent to the lines for purification and production of lOPs (Iron Oxide Pigments) according to known techniques.

The aqueous solution obtained after the precipitation of iron still contains manganese sulphate. There is also ammonium sulphate produced as a result of the conversion of iron sulphate into mixed iron oxyhydroxides.

In the process according to the invention, the aqueous solution thus described is treated to recover manganese (E) in a valuable form. In order to achieve this, ammonium bicarbonate is added to the solution, which causes the precipitation of manganese carbonate and the formation of further ammonium sulphate. According to a preferred embodiment of the process according to the invention, the ammonium bicarbonate can be obtained by carbonation of ammonia in solution obtained from the thermal pre-treatment section (B). The solution containing ammonium sulphate is sent to a dedicated section where solid ammonium sulphate and water are produced according to known procedures. Solid ammonium sulphate is used as an additive in the thermal pre-treatment step (B), whilst the separated water is directed to the head of the hydrometallurgy sections.

The manganese carbonate obtained is subsequently transformed into marketable manganese oxide by means of solubilisation and electrodeposition processes (F).

The residual solid obtained during the solubilisation of iron sulphate and manganese sulphate in the hydrometallurgical treatment step (C) still contains silver as a valuable element and is directed to the dedicated recovery cycle (G). In particular, the material is placed in contact with sulphuric acid in an aqueous solution and agitated in order to facilitate the passage into solution of any interfering elements for the recovery of silver. Two flows are obtained from the solid/liquid (H) separation. The aqueous sulphuric acid based solution solubilises any remaining iron and manganese residues present on the solid and is fed back to the iron and manganese recovery line (D).

The remaining solid, containing silver, is leached (I) with a concentrated sodium chloride solution to obtain a soluble silver polychloride (hereafter referred to as silver chloride).

According to an embodiment of the process according to the invention, the temperature at which leaching (I) is conducted is between 25 °C and 50 °C. The silver is subsequently recovered in the form of silver sulphide by adding sodium sulphide to the previously obtained silver chloride solution (L).

The solid residue produced at the end of the hydrometallurgical processes described is a chemically inert material, based on calcium/aluminium silicates, suitable for use as a secondary raw material in civil engineering and construction works. In the less favourable case that the market is unable to fully absorb this material, the remainder complies with the requirements for disposal in landfills, without further treatment.

The proposed process, as a whole, is therefore able to produce, from the waste originally destined for landfill, compounds with high added value destined for the reference markets (IOP, Iron Oxide Pigments), manganese, silver, aggregates for use in civil engineering and construction works), reducing the potential waste to be disposed of; in a less favourable scenario, where it is not possible, for market reasons, to reuse the inert fraction, it can be easily disposed of. Even in this unfavourable case, the weight reduction of waste to be disposed of, compared to the current situation, would be at least 50%.

The following example describes an embodiment of the process according to the invention.

Example

The example illustrates in practice an application of the process according to the invention.

A form of application of the process according to the invention to the treatment of a Waelz slag produced by the zinc production line according to the Waelz process is illustrated.

Table 1 below shows the elementary analysis of the Waelz slag used in the test.

Table 1

The Waelz slag used in the test is a cooled slag that has been stacked in a heap and ground to increase the homogeneity of the material. In the example described, a 1 :1 mixture is created between ground Waelz slag and solid ammonium sulphate. This mixture was subjected to thermal pre-treatment in a neutral/oxidant environment at a temperature of 400 °C for 2 hours. In this application, the thermal pre-treatment reactor used is an electric muffle furnace. During the thermal pre-treatment ammonia production was observed, but it was not recovered in the example described. At the end of the pre-treatment of the Waelz slag, iron sulphate and manganese sulphate were formed, together with other sulphates of the main elements present, as can be seen from the data in Table 2, which refers to the crystallographic composition of the pre-treated solid. Table 2 The resulting material was subjected to hydrometallurgical treatments.

In this example application of the invention, the material produced at the end of the Waelz slag pre-treatment was then subjected to leaching with water, using a liquid/solid (L/S) ratio of 10 and a contact time of 1 hour at a temperature of 25 °C. A stirred reactor at atmospheric pressure was used for leaching. At the end of the specified time interval, the residual solid was separated from the aqueous solution by vacuum filtration. The resulting solid was dried in a thermostatically controlled oven at 70 °C while waiting to undergo silver recovery treatments.

Table 3 below shows the composition of the aqueous solution obtained after the aqueous leaching of the thermally pre-treated Waelz slag.

Table 3 Compared to the initial Waelz slag, a selective extraction of iron and manganese of 46.8% and 44.0%, respectively, was achieved following water leaching.

The aqueous solution thus obtained was subjected to a pH correction until a pH of 4.5 was reached by the addition of ammonia in order to selectively provoke the precipitation of the iron compounds as oxyhydroxides, which were separated by filtration and further heat treated to obtain lOPs (Iron Oxide Pigments). At this stage, the precipitation of iron originally in solution is greater than 99%. As a result of the pH change, ammonium sulphate in solution was also obtained. After separation of the iron hydroxides, the aqueous solution, containing the manganese, in addition to the ammonium sulphate, was subjected to hydrometallurgical treatments to recover it. The manganese was then converted to manganese carbonate, bringing the solution to pH 7 by the addition of ammonium bicarbonate and, subsequently, separated by precipitation. The manganese carbonate thus obtained can then be converted into manganese oxide by electrodeposition after solubilisation in acids.

During the manganese precipitation phase, additional ammonium sulphate is formed in solution. The resulting liquid is sent to the solid ammonium sulphate recovery cycle by thermal processes. These give rise to solid ammonium sulphate and water, which are reintroduced in steps (B) and (C) respectively.

The residual solid obtained in (C) from the leaching of the Waelz slag was sent to the head of the silver recovery cycle (G).

In this section, it was leached with a 0.1 M sulphuric acid aqueous solution at a f L/S (liquid/solid) ratio of 10 for 1 hour at 45 °C, in order to concentrate the silver in the solid fraction and eliminate any interferents. The resulting liquid, also containing iron and manganese, is sent to step (D). By leaching the silver-enriched solid with a 5M sodium chloride salt solution, a silver extraction in solution of 60% was obtained. By addition of sodium sulphide to saline solution containing silver, over 99% silver was recovered as silver sulphide by precipitation.

The invention is described by way of example only, without limiting the scope of application, according to its preferred embodiments, but it shall be understood that the invention may be modified and/or adapted by experts in the field without thereby departing from the scope of the inventive concept, as defined in the claims herein.

Claims

1. A process for the treatment of residues of the non-ferrous metallurgical industry, in particular Waelz slag and residues deriving from thermal fuming processes of jarosite, goethite or KSS slag, comprising the following steps:

- thermal pre-treatment (B) by the addition of ammonium sulphate (or nitric acid and/or sulphuric acid mixed with ammonium sulphate or with another reagent capable of promoting a reaction with the Waelz slag which makes it possible to obtain firstly the formation of the reaction intermediate ammonium hydrogen sulphate, (NH4)HSC>4 and, subsequently and in sequence, the formation of soluble iron and manganese sulphates and the development of ammonia) and gradual increase in temperature from ambient temperature to a final temperature in the range 200 °C to 600 °C, in a neutral/oxidising environment, to obtain the conversion of the iron, and manganese if present in the material, into compounds such as iron sulphates, and, possibly, manganese sulphates, soluble in water, with the formation of ammonia; and

- hydrometallurgical treatments (C), according to the following sub steps: - addition of water and mixing,

- correction of acidity up to a pH of 2 to 8 in an oxidising environment, to obtain the precipitation of iron in the form of mixed oxyhydroxides.

2. The process according to claim 1 , characterised in that it comprises a step which precedes said thermal pre-treatment step (B):

- initial pre-treatment (A) of the material to be treated, to obtain homogeneous particle size characteristics.

3. The process according to claim 1 or 2, characterised in that said step of correcting acidity is achieved by addition of ammonia obtained from the thermal pre-treatment step (B).

4. The process according to any one of the preceding claims, characterised in that said oxidising environment is achieved by insufflation of oxygen, air or air enriched with oxygen.

5. The process according to any one of the preceding claims, characterised in that said precipitation of iron is obtained at a temperature of between 10°C and 150°C, with a partial pressure of oxygen of between 0 bar and 10 bar and a contact time of between 1 h and 4h.

6. The process according to any one of the preceding claims, characterised in that the solution obtained after precipitation of iron is added to ammonium bicarbonate for causing the precipitation of manganese carbonate and formation of additional ammonium sulphate.

7. The p process according to claim 6, characterised in that said ammonium bicarbonate is obtained from the carbonation of ammonia obtained in the thermal pre-treatment step (B).

8. The process according to any one of the preceding claims, characterised in that the ammonium sulphate which can be obtained by separation from the solution obtained after precipitation of iron and, if present, of manganese, is used as an additive in the thermal pre-treatment step (B).

9. The process according to any one of the preceding claims, characterised in that it comprises further steps for the recovery of any silver present in the residual solid obtained during the solubilisation of iron sulphate and manganese sulphate.

10. Use of the iron precipitated in the form of mixed oxyhydroxides in the process according to any one of claims 1 -9, after separating from the solution and purification, for the production of pigments based on iron oxides.

11. Use of the manganese carbonate obtained in the process according to any one of claims 1 -9, for the production of manganese oxide.